1. Field of the Invention
The present invention relates to a recording head having a nozzles, a droplet discharge device for discharging droplets from a nozzle while moving the recording head back and forth, and a droplet discharge method using the recording head.
2. Description of the Related Art
Recently, a droplet discharge device that forms an image by discharging droplets while moving, back and forth in a main scan direction, a recording head that forms dots that compose an image on a recording medium, and by relatively moving at least one of the recording head and the recording medium in a sub scan direction orthogonal to the main scan direction, has become common.
Further, for the recording head, a recording head in which plural nozzles are arranged in a two-dimensional fashion (matrix type recording head) in order to densely arrange a number of nozzles within a limited nozzle surface area is well known, and in image formation using a matrix type recording head, because droplet discharge by densely arranged nozzles is possible, a high-quality image may be obtained at high-speed. For example, in a case of image formation using a matrix recording head with numerous high-density nozzles as shown in FIG. 17, high-density and multiple droplet discharge in the sub scan direction is possible by means of a single main scan, so that a high quality image may be obtained at high speed.
However, in a droplet discharge device using a matrix type recording head, there has been a problem whereby a formed image is susceptible to concentration unevenness in the sub scan direction.
In order to solve the problem, Japanese Patent Application Laid-Open (JP-A) No. 2004-90504 discloses a technique to decrease the visibility of concentration unevenness by increasing the spatial frequency of the concentration unevenness in the sub scan direction by mixing dots having a large diameter and dots having a small diameter and by forming the dots at constant intervals.
Further, JP-A No. 2007-29786 discloses a technique in which the recording head has a nozzle group composed of a first nozzle line at an angle φ with respect to the main scan direction and a second nozzle line at an angle −φ with respect to the main scan line and intersecting to the first nozzle line, and the nozzle lines are arranged in an X-shape such that adjacent nozzles belong to different nozzle lines when the nozzle group is projected in the main scan direction so as to align the projected positions of all the nozzles in a straight line, thereby decreasing the visibility of concentration unevenness occurring in the formed image.
JP-A No. 2002-103579 discloses a technique to prevent a striped pattern from occurring in the sub scan direction by recording the image by making two adjacent rasters formed on the recording medium by the same nozzle serve as one unit in the forward movement and backward movement in the main scan direction in a recording head having plural nozzles arranged in a zig-zag manner along the sub scan direction.
However, in the technique disclosed in JP-A No. 2004-90504, in the recording head in which plural nozzle groups, which have plural nozzles arranged in a straight line that is inclined with respect to a moving direction at a predetermined angle, are arranged in the sub scan direction such that the intervals in the sub scan direction between nozzles adjacent in the sub scan direction are substantially identical and the positions in the sub scan direction of the nozzles belonging to the respective nozzle groups are substantially aligned, there is a problem that the concentration unevenness in the sub scan direction cannot be reduced, the concentration unevenness occurred due to difference in density of the dots that is formed by the adjacent nozzles between nozzle groups adjacent in the sub scan direction and the dots formed by the nozzles belonging to the respective nozzle groups by the recording head inclined by inertia force in accordance with the motion in the main scan direction. Also, in the technique disclosed in JP-A No. 2007-29786, there has been a problem that the nozzle arrangement is complicated and the design of the flow path for feeding the nozzle with the ink liquid is restricted. In addition, in the technique disclosed in JP-A No. 2002-103579, there has been a problem that the technique sometimes may not be applied to the recording head with high-density nozzle arrangement.
Also, the image formation by using the matrix type recording head has a problem that the concentration unevenness in the sub scan direction easily occurs due to the displacement of the head posture. For example, as shown in FIG. 17, although the intervals between adjacent dots in the sub scan direction are constant when the matrix type recording head is correctly attached at a predetermined angle with respect to the main scan direction, in a case in which there is a slight error in angle in the head posture, the interval a between dots adjacent in the sub scan direction in the nozzle arrangement and the interval b between dots adjacent in the sub scan direction at a folded portion of the nozzle arrangement differs to each other, and a high concentration portion and a low concentration portion occur at an interval of folded pitch P of the nozzle line, so that the concentration unevenness in the sub scan direction is visually recognized.
In the above-described documents, the method to effectively reduce the concentration unevenness occurred at the folded portion of the nozzle line is not disclosed.